Process for production of magnesium compounds



Sept 14, 1948- G. H. GLoss ET A1. 2,449,293

PRocESs'VFoR' PRoDUcToN oF MAGNESIUM COMPOUNDS Y Filed Mayl 2, 1944 3 sheets-sheet 1- io form large crgsrals cfm-CO2 conlraning gas' Mgcoy 3H2o ATTORN EY Sept. 14, 1948. G. H. GLOSS ET AL 2,449,293 PROCESS FOR PRODCTION OF MAGNESIUM CMPUNDS Filed AMay 2, 1944 s sheets-sheet s Sea wafer Corirrolled carbonaion Clarified brine ro form large slzed crgsials 9/ Malxgnesuyn containing mineral luke dcomH-e ormagnes'd'e Eri que# i ng /04 Carbon Carbohermic proc ess Mg Residue seHlemenfl oF .Mgpuzlurrg .Overflow Lime slurrg .Mg(OH)2. slurry 93 I Carbonaion forming Washng Mgcos. SH2@ INVENTORS @UNTER H. 61.055 B 5061,? B. BAKER UM am ATTORNEY UNITED STATE Patented Sept. 14, 1948 PROCESS FOR PRODUCTION OF MAGNESIUM COMPOUNDS Gunter H. Gloss, RedwoodwCity, and Edgar B. Baker, Burlingame, Calif., assignors to Marine Magnesium` Products Corporation, South San Francisco,` Calif., a corporation of Delaware applicati@ May 2, 1944,seria1No. 533,704

This invention relates generally to processes for the production of magnesium Acompounds fromvarious magnesium containing materials.

In the past many methods have been utilized for the purpose of producing magnesium` compounds from various magnesium containing minerals such as dolomite, magnesite, and brucite. All of these processes have definite limitations with respect to cost `of production and the purity of the magnesium compounds which can be produced. Either they have been unable to produce magnesium compoundshaving a purity of say 95 to 98%. or if such purities have been `attained then the cost of production has been excessive. Furthermore the plant equipment required has been expensive and` elaborate, thus adding to capital investment and cost of production.

It is an object of the present `invention to provide a methodof the above character which will go beyond the limitations of prior `art processes. Particularly the present method makes possible production of purer magnesium compounds at lower cost. In addition it makes possible high recoveries and high` capacity production of magnesium` compounds from magnesium containing raw materials -without an undue capital-investment for equipment. V,

Another object of the invention is to provide a controlled "carbonating procedure making possible production of large size neutralmagnesium carbonate crystals, whereby such large; size particles can be readily separated by hydraulic classification from other solid phase material, such as calcium carbonate and the like.

Another object of the invention `is to provide a process `for producing magnesium compounds which will not consume large amounts of chemicals, and which will notrequirelarge quantities of fresh water, such as is necessary inmany prior processes. e i n A further object of` the invention is to provide a process applicable to a wide varietyof minerals or other magnesium containing mixtures. T In this connection the present invention can be applied to virtually all dolomitic minerals, whereas` it is well known that some prior art mechanical separation methods are applicable onlyto certain types of dolomitic minerals.` .i

Additional objects of the invention -wi11 ,appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawing.

- monies. (cl. zs-cv) Figure 2 illustrates a piece of equipment which lcan be used for the` carbonating operation. Figure 3 is a flow sheet for carrying outthe irivention on materials like` dolomite, and which produces a relatively pure magnesium compound.

p Figure 4 is a ow sheet illustrating a procedure for the treatment of a relatively granular form of calcineddolomite. v j

Figurep is a flow sheet illustrating a further embodiment, involvinga preliminary removal of a part of the calcium content prior to treatment .forremoval of the magnesium content Figure 6 is a flow sheet illustrating another embodiment of the invention in which the source of magnesium s sea water or like brine containff` ing convertible` magnesium salts.. Figure '7 is a flow diagram illustrating another embodiment likewise utilizingisea water as" a source of magnesium.

Figure Sisa iiow sheetillustrating another elm.- bodiment in which residue froma thermal reducti-onprocess is utilizedas a source of magnesium. The present invention is characterized `by a particular type of` controlled carbonationcarried out on a slurry containing magnesium oxldeor magnesium hydroxide or both, and which` carbonation serves to form large `sized crystals of neutral `magnesium carbonate (MgCOsHeo). Referring rst to Figure 1 a nriagnesium` oxide containing material is shown being subjected to slakingat. lll, whereby slurry is `produced containing' magnesium` oxide, hydroxide. or both. The `magnesium oxide containing material can be produced by calcining a wide variety of materials and mixtures, as for example dolomite, magnesite, brucite,1serpentine, or residues having a magnesium content which will be `converted to magnesium oxide upon i calcining. Slaking can be accompanied `by grinding to provide alparticle sizesuch that the bulk of the particles are less than 15 `microns in diameter. This particle size is desirable in order to make feasible the subsequent classiiication step` In step I I the slurry is T subjected tocontrolledcarbonation to form large sized crystals Iof neutral magnesium carbonate (MgCOeHzo). The `material is then subjected toa ,separating operation l2 whereby thetneutral magnesium carbonate is separated outfrom other solid phase impurities` which for example may include calcium carbonate, silica, etc. n

`Controlled carbonationat I l can be carried out either `as a batch operation, or continuously. `In ordinary carbonation of magnesium oxideorlhyfdroxide neutral magnesium carbonate is prof trast to a particle size of say to 40 microns,

with ordinary rapid carbonation. .According to our discovery the desired large `'partiflle size can be secured when the rate of conversion of magnesiu'm oxide or hydroxide intothesolid phase.-

neutral magnesium carbonate is not in excess of 0.12 mol per gallon per hour, and Apreferably somewhat slower. In practice the rate of conversion is controlled by-maintaining the concentration of the liquid phase (total CO3 and HCOa ions between certain upper and lower limits. According to 'our observations good results `are secured whenr the .concentration of the liquid phase at no time falls below about 0.02 mol per liter, and does not "exceed'0Q05 mol per liter. 'When control-led in this manner the bicarbonate concentration may fall Withinthe `limits of about `0.0.1 to.0.015 mol per. liter.

To carry vout batch carbonation with control as described above,`apparatus such as illustrated in Figure 2 can be employed. This consists of a simple tank I3 which contains an initial quantity lofthe slurry to loe carbonated, and which has a discharge pipe I4 at its "lower end.V Pipe 'I6 is 'forthe purpose'ofintroducing the slurry, and pipe I'lv serves tointroduce a'oarbon dioxide containing gas, such as'flue gas. lIntroduction of "the'i'lue gas will generally effect sucient agitation of the material, or if`thegas'is-relatively concentrated with yrespect. V'to its vcarbon dioxide content, slow mechanical agitation can be employed. T o carry outa carbonating operation an initial batch ofthe slurry is introducedv into' the 'tank "i3, and this batch is then seeded=with a small amount of previously produced neutral ,magnesium carbonate. TheA solids concentration oftheslurry may vary from say 0.5 to 1.5 moles 'perliter'ofMgQ It is desirable to introduce a sufficient quantity of'the seed crystals to prevent `the formation 'of vnew small crystals Vduring `the initial sta'gesof the carbonation. `Elue-gas is now introduced'intothe mass of material through pipe l1 at a relatively slowrate, and continued until all of the magnesium hydroxide of ,the batch has been converted to ysolid phase neutral `magnesium carbonate. At this time introduction of further slurry through pipe v'lliiis commenced `and is maintained 'continuously inubalance-with the ow of flue gas. This balanceis suchtliat 'the concentration of the liquidphase is'kept-b'eltween the: limits of 0.02 to 0.05 'mol per `liter,' as exl'ilainedabove.` Carbonation vis continued underlsuoh `conditions'unti-l the'carbonating vessl 'isi'lled, at which 'time'the charge is drained off through'the pipe i4. Instead'of draining off all ofthe charge through pipe I4 itis feasible to bleedfoi the solid phase material at a controlled yrate,'whereby the'process is operated 'continuously. In general the carbonat-ing equipment should lbe designed 'and operated whereby. it provides a retentio-n period of about 36 to 48 hours for theffeed slurry. The'entire carbonating 'operation'is carried out under normal atmospheric temperaturesjranging for example from-10 to As previously stated the neutral magnesium carbonate is of relatively large particle size and can be readily separated by suitable known methods, such as hydraulic separation or classiication, from other solid phase impurities such as :calcium carbonate, silica, silicates and the like. The removed neutral magnesium carbonate can then be treated to form other magnesium compounds, as for example it can be calcined to form magnesium oxide, or heated to an elevated temperature to convert it to basic magnesium carbonate.

AIn the slaking operation I0 the temperature of the slurry may rise to the boiling point due to the hydration of t-he calcium oxide, but by the time the latter reaction is completed the less reactive magnesium oxide may be only partially hydrated.

It is possible to extend the period of slaking until all of the magnesium oxide is hydrated, lbut the slurry being introduced into the following carbonating operation-l1 may contain either magnesium oxide or hydroxide or a mixture of both.

The purity ofthe magnesium compound-produced'by the process ofFigure 1 is dependent partly upon the proportion of magnesium oxide to other compounds present in the material un-` dergoing treatment. VIf there is a relativelyhigh limecontent in the material, as with dolomite, lsome calcium -carbonate will occlude with the crystals of neutral magnesium carbonate, thus affording a lim-e contamination. Where 'relatively pure magnesium' compounds are required, and dolomite is being utilized as a source of raw material, we can make use of the two stage carbonating process shown in Figure 3. In this instance dolomite is subjected to complete calcining atl2`| and s-laking at 22 toformva slurry containingboth magnesium oxide and calcium hydroxide. As will Abe presently explained it is de- 'sirable to yuse asolution of sodium carbonate for this step, in: place of fresh water, with conversion ci a minor part-of the calcium oxide to calcium carbonate. The slurry is-reacted at `23 with a solution of vsodium carbonate whereby the calcium hydroxide is converted to calcium carbonate and the sodium carbonate is converted to sodium hydroxide. This Amaterial is then subjected to separation Ilat l'24, as by centrifuging'whereby the sodium hydroxide yis removed in liquid phase and Awhereby the resulting centrifuge cake contains calcium carbonate and magnesium oxide or hy- Adroxide or'both. This-centrifuge 'cakeis then diluted -with water at`26 to form a slurry, and the slurry is subjected to controlled carbonation at "F'ZLthe'same as the-controlled carbonating lstepf-l II lo'rfligure""1.A 'In this controlled carbonating'operationthe magnesium oxide or hydroxide yis converted to large sized crystals of neutral magnesium carbonate. The carbonated material is then subjected to suitable separation treatment lati :29-1as'7by hydraulic vor centrifugal classification, whereby 4'the fastsettling neutral magnesium carbonate-readilyseparates out 'as an underflow,

and-thecalcium'carloonate and other solid impurities are removed in'the overflow. Water can vberemovedfrom `the overflow inthe thickening `.operation `213 and reusedf'backr in the process, 1as

thefseparating' operation 2li,l as indicated by line 3|.-'1In this reaction the magnesium carbonate is `converted to magnesium hydroxide, whereas then subjected to dilution at 34, preferably after grinding or violent agitation to `break up ag` glomerates, and theresulting slurry then subjected to controlled carbonation 35. This conu trolled-carbonation is the same as that previously described, to again produce large sized particles of neutral magnesium carbonate. lCarbonated` material from this operationis then subjected to suitable separation treatment 35 as by hydraulic or centrifugal classification, whereby the'neutral magnesium carbonate is removed in an underflow and is thensubjected to the thickening and dewatering operation 31. `The dewatered mate-` rial `can then be calcined at 39 toform magnesium oxide. The overflow from classifying operation 36 can be subjected to thickening atti whereby the eiuent is made available for use back in the process, as in the diluting operation 34 as indicated.A

l In the second controlledcarbonating operation 35 the neutral magnesium carbonate produced is relatively free of `occluded calcium.

Therefore the final magnesium compound produced from this carbonate is of relatively high purity.

U In the process of Figure 3 it is feasible to operate the rst carbonating operation 21 and subsequent separation 29 whereby the overflow from 29 contains about 85 to 90 mol percent of the total calcium, and about to 20 mol percent of the total magnesium, 'with the underow pass-` ing tooperation 28 containing 80 to 90 mol percent of the magnesia and 10 to 15 mol percent of the calcium.` After the second carbonating operation the overflow from the separating vor classifying operation 36 can contain from 8 to `13 mol percent of the total initial calcium and 4 to 6 mol percent of the total magnesia. The nal magnesium oxide from the calc-ming operation 39 can contain for example about 3% calcium oxide, together with small amounts of impurities such as silica and iron, depending upon the composition of the original dolomite. With a typical type of dolomite containing after calcination 40% magnesium oxide and 55% calcium oxide, the amount of neutral magnesium carbonate obtained (MgO` basis) is about '75% of that originally present in the calcined dolomite and the purity is of the order of 96% or better.

Sodium carbon-ate is a relatively inexpensive chemical for use Iin the process of Figure 3. In place of this alkali carbonate, it is possible to use potassium carbonate. The concentration of the sodium carbonate solution 33 is maintained within `suitable limits, such as of the order of 1 mol per liter, with occasional additions of sodium carbon-ate to compensate for losses from the systern. In general however the consumption of sodium carbonate will be negligible.

Cil

Some calcined dolomites are relatively granu, Tarso that .after hot slaking and agitation a,

substantial amount of `material remains in the form of hard agglomerates which are difficult to ccmrninute to a particle size of 5 microns or less by grinding. Y Furthermore such a grinding operation would be expensive to thepoint of making .its use prohibitive. i When such granular materials are` encountered it is preferable to utilize aprocess such as shown. in` Figure `4, in which during the treatment the material is dividedL `into coarse and fine fractions which are separately treated. Thus the calcined dolomite inthis `instance is subjected to slaking at 5I, to`

forma slurry forthe reaction operation 52. Here theslurry is reacted with sodium `carbonate the same as reaction operation 23 of Figure 3. The

calcium content of the slurry is thus converted to calcium carbonate, withl formation of sodium hydroxide. After the reaction operation thema! terial is subjected tofthe separation operation 53,

as by centrifuging,` and the centrifuge` cakeis` then diluted at 54, and passed to the preliminary classifying operation l 55. This preliminary classification is carried-outby suitable hydraulic or centrifugal equipment, capable of maintain-` ing an overiiow of fine material, 'and an under-` The coarsefraction ory flow of coarse material. underilow contains the i aforesaid hard granular agglomerates.` This coarse material is thensubjected to conventional carbonation at 56, which can be carried out by contacting the material with ue gas under such conditions that` car-` bonation occurs very rapidly. During carbona-l tion the magnesium hydroxide occluded inithe granular agglomerates is extracted `and cone` verted into `separate small crystals of neutral magnesium carbonate. Due to the fact .that the carbonation` is carried out rapidlythe particle size of the neutral magnesium carbonateis relatively ne. The carbonated material `is then subjected to a classifying operation 51, which is carried out with` suitable equipment whereby there is maintained an overflow of finematerial, and an underflow of'coarse material.- The overflow consists mainly of fine particle `size neutralk magnesium carbonate, while the underow consists mainly of coarse or granular agglomer'ates The underow `can be subjected to thickening 58, thus affordingwater of4 calcium carbonate.

which can be reused backin the process, as for example for the dilution op-eration`54. The over-` flow from the classifying operation 51 is sent toy the thickening operation 59 for further treat-` ment as will be presently explained. i

The overflow of ne dolomitic material from the `preliminary classification 55 is subjected to controlled carbonation 3l as previously described,v

to form large crystals'of neutral magnesium'car-4 bonate. This carbonated materiall is then passed to `a classifyingoperation 62, which is carried out in suitable classifying equipment capable of establishing an overflow of ne material, and an process, as for example use in the diluting opera-V tion 54. i The thickened material from 59, whichv represents the fine and coarse neutral magnesium carbonates from;the classifying operations 51` 'further purification.

andi 6'2"; is. preferably? furtherE puriedi in: an series of stepsrincluding reaction at;64s with sodium'y hyr droxide. Sodium.. hydroxide for' this operation-f. Gambe?. obtained lay-utilizing eili-uentfiom the sep` aratin'g operation 53;v asf indicated. Adter` this reaction; thematerial iszsubjected to the separat,- ingsoperation` 65,:as by-centrifuging, .anda theleilui.-I ent;v from this operatiomwhich. contains sodiumi carbonate,v can be reusedfon the-reaction;opera-Yr tion.52:, asindicated. The centrifuge' cake from: operationv 65 is shownbeing diluted ataiito form a slurry.'4 for the controlled carbonation operationA 651. This operati-ouais carried out by, v the. procedure previously described'. in; orden' to producei large crystals ofl neutral magnesium carbonate. 'Ihe carbonated' materiali is passed' to thesepa-- rating operation 68,. which' canf be carried` out' by; suitable classifying equipment, forseparatingouti the desired, neutral' magnesium. carbonate- Thelun'derflow fromV the classifying operationis` shown being further treated*v by thickening: andi deWa-tering 69,1andr calcining 1.I.,.to:produce magnesium oxide. The overiiow,` fromv classifyingl operation 68is shown passing through the thickening' operation 1 22, .to produce an` effluent'v which. can belut'ilized asWa-ter in the dilution operation.'4 661.. Residues from both thickening operation; 63 and '12l contain calcium carbonate'together with other solldiphaseimpuritiessuch.; as silicay and the like; andvarying quantities of: residual neutral mag-l nesium carbonate;

In-both the processesfof Figures 31and'4'neutral1 magnesiumcarbonate is reactediwith sodium hydroxide', to? obtain al magnesiumhydroxideY forV Before reaction with sodium hydroxid'ee it ispossible to convert the new traie carbonate. to basiomagnesium carbonate,y by: application of heat.

In place' of` utilizing. two stages of:` controlled:y

carbonation to secure a relatively pure form: of' l;

neutral magnesium carbonate, iti is possible to: utilize preliminaryv treatment for." the' purpose of removing a substantial aniountf of Y thez lime? con-.- tent,V prior to treating; the remaining part. olf-l. the.A slurry for' removal of neutral. magnesiumv carA bonate. Thus'I referring; to Figure 5,v dolomite: is. shown being; subjected? to'y calcining 111, andy slak'- ing 18 to' form a slurry; containing magnesiumI oxide and. calciumi hydroxide; Ats19 this slurr-yf is treated for removal ofasubstantial: amount. of theA lime content. While. thevmethods employedf for this purposemay vary,,itisrpossibleffor exami-y ple toutilizealeaching processsucn as disclosed@ in? Patent No. '734,030l of 1903; to Young: Thus: according to the process of this patent'. vvaterisv utiligedftofleach out a substantial part. of; the lime content, .with withdrawal of a clariedlime solwr tieni as indicated', which isy subjectedf tor a. oar-f bonatingf operation` 8=Izto precipitate calciunrcarV bonate. Inza suitableseparatingioperation 84? this calciuml carbonate; or precipitated chalk, can be? removed. and the water: returne'df for leaching asi indicated.

The.w remaining solid` phase;V material fronuA ther operation 1'9, which accordingV to` Youngs-process) would.- be: averyl impure and practically? useless; grade of.Y magnesiagis then subjectedstothe: coney trolledY carbonation 83., which` is, carried out irrthef samermanner` asl previouslyl described.. Carlton.` atedimaterial `from thisoperation, which: contains large: crystals of. neutrali. magnesium. carbonate; isin then subjected. to the* separating operation. 814;A ifo; separate.i out the: desiredr neutrali magnesium carbonate; Bymean's oftheiprocess-.of Fig;- 5' the. amount.4 of calcium present during; the controlledi carbonation is; reduced whereby the amount-ofi' calcium occluded with the particles of: neutrali magnesium carbonate is materially reduced;V Thus with: such a combination" processit is possi-fv ble to produce-.af relativelygpure form ofneutralz` magnesiumcarbonate.

Figure 6y showsl application off the invention` to; a process-wherein magnesium compounds are dee.` rived from brines containingA convertible magg-.- nesi'um salts, such as sea Water. Thus` in*l this?l instance sea-water is subjected to apretreatment'; 8B primarily for the purpose oi removingy disi=A solved calcium bicarbonate.k Thisoperationl-ine.- volves-.reacting the brine withy small amounts; of?. lime or dolomite slurry as indicated,v and ca'nlbe. in; accordance with'. ClarkePatent 2,276,245.; The? procedure disclosed` in that patent involvesyuse; of a activated sludge which is continuously. re-h circulated into contact with the incoming brina and which aids in securing complete precipitartion of calcium bicarbonate as insoluble' calcium; carbonate. Sludgeisycontinuously bled oir fromi this circuit and consists of solid phase calciunrcarbonate and a considerable amount. ofprecipitated magnesiumv hydroxide, together with large; quantities of organic and inorganic impurities. In- Figure 6 this sludge is shown being passedto:l the controlled carbonation operation 81\, which. isi carried out in such a manner as to produce; largesized crystals of neutral magnesium carbonate;y The carbonated material is thenpassed.y tor the: separating operation 88, whereby after washing; as indicated a relatively pure form of;I neutral-I magnesium carbonate is removed. The' brine;v orA sea Water fromthe pretreatment-` operation 81' is -shownbeing passed to further treatment for reif moving; magnesium hydroxide. from` the same: This further treatment can include filter 89ffor the purpose ofv clarifying the.v brine, and* precipi tation and -settlement at 9| where the brineis: reacted withy lime slurry to precipitatemagnesium hydroxide. A precipitation procedure of= this: character is disclosedfor example'in'Chesny Pat-1:- ent No. 2,0895339fof 1937. The magnesium-hy'- droxide slurry recovered from operation 9| cam` be further purified -by washing.

A dolomite slurry, that-is one'forrnedby: slakingfl a calcined. dolomite, is preferable for pretreat` ment86 because its use results in ahigher perf centage of magnesium hydroxidein the resultant. sludge.`

Figure 7. illustrates another procedure-inr lwhich, the invention is utilized in-conjunctionwith pro,-l ductionof magnesium compounds-from sea Water; The sea watery in thisinstance requires no pretreatment and isshown being subjected directlyL to precipitation with limeor. dolomite at 9.2;.withf settlement of the precipitated magnesium hy droxideand other solids: to. form a slurry. Ther linie or'calcined dol-omite used torprecipitationl in this case canE be either inslurry formv or in. the for-rn of a dry` powder, and it may contain5 a relatively highpercentage of impurities. Theg impuremag'nesium hydroxide slurry is then, passed to the controlled carbonation operations S3. which canV be carried out bythe controlled pnocedurepreviously described, to produce large. crystals of. neutral magnesium carbonate. Thecarbonated material. from 913 is then passedA to: the separating operation 94 for removal4 of' the; desired neutral. magnesium carbonate; whiche can then be washed as indicated. The overflow.' from 941: contains ther impurities, whichzincludei calcium carbonate'. As inl the other proceduresE the removed neutral; magnesium: carbonate can;

-besubjected to further treatment for 4the production of yarious' magn-esium compounds.

With respect to the type of materials which can be utilized for the production of magnesium compounds, `further 4reference can be made to various residue mixtures containing magnesium oxide or other magnesium containing-compounds. For example we can employ the residueresulting from the carbothermic process for the manufacture of magnesium metal, which consistsofunreacted or back-reacted magnesium oxide,y carbon, and other solid impurities. Such a residue can be treated in accordancev with the procedures previously described tor the production'of la relatively pure form of magnesium oxidexl Where dolomite is being utilized as "a `source of i magnesium oxide for use in the carbothermic process, it is possible to utilize a complete` process such as shown in Figure 8. Thus in thislinstance a magnesium containingmineral like dolomite or magnesite `is subjected to calciningl andthe calcined material is then subjected tot slaking 98. The slaked material is then subjected to the contr'olled carbonating operation 99 for the production of large crystalline particles 'of neutral mag nesium carbo-nate. The carbonated material is subjected tothe separatingl operation `I 0 I 1 `for the removal of the desiredv neutral` magnesium carbonate, and this material is then dried and calcined at |02, to form-magnesiumnoxide. vThe magnesium oxideI i'slbriquetted at |03 with carbon, and these briquets are then supplied to the carbothermicprocess |04. In thecarbothermic process the briquets are heated to an elevated temperature for reducing l the magnesium. oxide with carbon to form magnesium metal, after which the impure powder is retorted. There results from the carbothermic process a residue containing considerable residual unreacted or back-reacted magnesium oxide and carbon. This f residue can then be retreated for thewremoval ofits magnesium content, asuby re'turning'the same to the slaking operation 98. It will `be apparent that return ofthe residue in this fashion will result in removal of carbon in the separat- `ing operation IOI. This process is preferable to one in which part or all of the retort residue `is directly reintroduced bef-ore the briquetting` operation l.; TheA latter method tends to build `up impurities in the cycle and l,causes difficulties `in the briquetting operation because of the inconsistent composition ofthe `residue and other factors. If magnesium carbonate is recovered from the retort residue itself by controlled carbonation as described above, the overflow from the classifying operation consists essentially of iinely divided carbon which can be driedland used as a fuel for calcinationpf'the magnesium `carbonate.

In general our invention utilizes simple equipment whereby large capacity can be obtained without undue capital investment. It is applicable to a wide variety of raw material sources, and therefore plants using such a process may be located close to available markets.-

The copending and co-assigned application of Edgar B. Baker, `Serial No. 551,331, filled August 26, 1944, now Patent No. 2,442,481, is directed to similar subject matter and reference herein is accordingly made thereto.

We claim:

1. In a process for separating out a magnesium compound from an impure hydrous slurry containing o-ne or more magnesium compounds from a group comprising magnesium oxide and magnesium hydroxide, the steps of subjecting a mass of the slurry to contact with a carbon dioxide containing gas, continuously supplying l both said slurry and carbon dioxide containing gas to the mass undergoing treatment, controlling the rate of introduction of the slurry and `the carbon dioxide containing gas to the` mass undergoing treatment so as tomaintain the'rate of conversion of the magnesium compound to normal magnesium carbonatenot in excess oi about 0.12 mol pergallon per hour, and the concentration of the carbon dioxide and carbonio acidin .the lid,- uid phase in the slurry between the limits `of 0.02 to 0.05 mol per liter, such carbonation servingto produce particles `of neutral magnesium 'carbonate of the order of. 200 to400 microns in size and 'separating' out the neutral-magnesium carbonate from the `other materia-l of the slurry.

2. A process asin claim 1 in which the initial slurry is one formed by subjecting sea waiter `con-,- taining convertible magnesium saltsV toi pretreatmentfor the `purposeof removing impurities and soluble `phase ,calcium bicarbonate,` the pretreatf ment involving reaction of the sea water withcalcium hydroxide whereby the calcium content of the `sea water is precipitated as solidwphasecalcium carbonate` and wherebya portionofgthe convertible magnesium `salts is:` precipitated as magnesium hydroxderthe Vslurry being obtained as a sludge containing `the magnesium hydroxide and calcium carbonate precipitated fromtlthe sea water during saidpretreatment.

8'. A method :as inclaim 1 in Which'the'initial slurry is one `obtained `by reactngbrine contain,- ing. convertible l magnesiumsalts with .calcium hydroxide to l substantially completely, precipitate the convertible magnesium salts .fas magnesium hydroxide, and then permittngwthe precipitated 4material to settlefto form lthe1slurry." t i,

`the same, said last named stepl being carried tout by controlled'carbonation `so as to,` ormlarge crystalsV of neutral magnesium carbonate `which are separated out from the slurry `of other solid phasenlaterial,v subjecting the remaining slurry of solid phasematerial tofwreaction with alkali metal hydroxide so as to convert themagnesium carbonate content of the same to magnesium hydroxide, and'. then carbonating the slurry so asto produce additionallarge lsize crystalsof neutral magnesium carbonate.

5. In a method of the character described, the steps of slaking a material containing both magnesium, and calcium oxides to form a slurry, reacting the slurry with an alkali metal carbonate to convert the calcium content to calcium carbonate, separating the solid phase material of the slurry from the liquid phase which .contains alkali metal hydroxide, subjecting the solid phase mai terial in slurry form to controlled carbonation to cause the bulk of the magnesium content of the same to be converted to large crystals of neutral magnesium carbonate, separating out the large crystals of neutral magnesium carbonate from the remaining material, causing the removed neutral magnesium carbonate to be reacted with alkali metal hydroxide whereby the magnesium content of the same is converted to magnesium hydroxide, removing the resulting solids from the liquid phase material, subjecting a slurry of the solids to controlled carbonation to again form large crystals of neutral magnesium carbonate, and then separating out the last named neutral magnesium carbonate from the other solids.

6. In a method of the character described, the steps of slaking a material containing bothmagnesium and calcium oxide, thereby forming a slurry, reacting the slurry with sodium carbonate whereby the calcium content is converted to calcium carbonate, and dissolved sodiium hydroxide is formed by the reaction, subjecting the material `to separation whereby the dissolved sodium hydroxide is'removed from the solid phase material, the solid phase material including calcium carbonate and magnesium hydroxide, forming a slurry of the solid phase material, subjecting the slurry to carbonation controlled to form 'large particles of neutral magnesium carbonate,

separating the neutral magnesium carbonate from other solid phase material, whereby the separated fine solid phase material contains calcium carbonate and other solid phase impurities, lsubjecting the removed neutral magnesium carbonate to reaction with sodium hydroxide, the sodium hydroxide being obtained from the rst named separating operation, the reaction serving to convert the neutral magnesium carbonate to magnesium hydroxide, thickening and washing the resulting slurry whereby sodium carbonate is removed in liquid phase and whereby the remaining solids consist of magnesium hydroxide with solid impurities, forming a slurry of the remaining solids, subjecting such slurry to controlled carbonation to produce large crystals of neutral magnesium carbonate, and then separating out said last named neutral magnesium carbonate to form a relatively pure magnesium compound, the Vsodium carbonate removed in the aforesaid thickening and washing operation being used in the rst named reaction of the initial slurry with sodium carbonate.

'7. In a process of the character described, the steps of slaking calcined dolomiteof granular type with water to form a slurry for further treatment, reacting the slurry with solium carbonate so that the' calcium content is converted to calcium carbonate, and whereby dissolved sodium hydroxide is produced as a result of the reaction, separating out the dissolved sodium hydroxide from the remaining solids, forming a slurry of the remaining solids for'further treatment, submitting such slurry to classification whereby a ne fraction is caused to pass out in an overow and a coarse fraction in an underflow, treating the overflow slurry to controlled carbonation to form relatively large size particles of neutral magnesium carbonate, said controlled carbonation being carried out by subjecting a mass of the overflow slurry to contact with a carbon dioxide containing gas, continuously supplying both said overflow slurry yand carbon dioxide containing gas to the mass undergoing treatment, controlling the rate of introduction of the slurry and the carbon dioxide containing gas to the mass undergoing treatment to maintain the rate of conversion of the magnesium compound to normal magnesium carbonate not in excess of about 0.12 mol per liter per hour and the concentrate of the carbon dioxide and carbonio acid in the liquid phase' in the slurry between the limits of 0.02 to 0.05 mol per liter, such carbonation serving to produce particles of neutral magnesium carbonate of the order of -200 to 400 microns in size, removing the neutral magnesium carbonate from other solid phase material after saidcontrolled carbonation, subjecting the underflow containing the coarse fraction from the classifying operationto carbonation to produce neutral magnesium carbonate of relatively small particle size, subjecting the carbonated material to separation whereby the small particle size neutral magnesium carbonate is removed in an overllow, and then merging said last mentioned voveriiow with the large size crystals of neutral magnesium carbonate for further treatment.

Y GUNTER H. GLOSS.

EDGAR B. BAKER.

REFERENCES CITED The following references are of record in the le of thispatent:

UNITED STATES PATENTS Number Name` Date 539,889 DAndria May 28, 1895 734,030 Young July 21, 1903 1,415,391 Rafsky May 9, 1922 1,449,696 Pike May 27, 1923 1,505,202 Judd Sept. 19, 1924 1,573,632 Crowell Feb. 16, 1926 1,863,966 Brosche June 21, 1932 2,012,854 Hill Aug. 27, 1935 2,066,066 Brooks et al Dec. 29, 1936 v2,139,934 Chesny Dec. 13, 1938 2,140,375 Allen Dec. 13, 1938 2,198,223 Muskat et al Apr. 23, 1940 2,211,908 OCormol Aug. 20, 1940 2,358,818 Miller Sept. 26, 1944 FOREIGN PATENTS Number Country Date 176,785l Great Britain Jan. 18, 1923 447,246 Great Britain May 14, 1936 

